As used herein, the term “(Ga,Al)N” denotes all members of the GaxAl1-xN (0≦×≦1) material family. The term “AlGaN” denotes a member of the GaxAl1-xN material family for which x is non-zero but is less than one.
Many electronic devices and optoelectronic devices are based on the (Ga,Al)N material family. These devices require at least one interface between an n-type doped material and a p-type doped material, in order to form a p:n junction and/or allow injection of electrical carriers into the device. GaN and AlGaN are both naturally an n-type doped semiconductor material, and p-type doped GaN or AlGaN is obtained by introducing a suitable dopant species during the GaN or AlGaN growth process. Magnesium is often used as a p-type dopant for GaN and AlGaN. Many devices require a free carrier concentration in the p-type doped GaN or AlGaN of at least 1018 cm−3, however, and there have been difficulties in obtaining such carrier concentrations in magnesium-doped GaN or AlGaN. It is relatively straightforward to incorporate magnesium atoms into GaN or AlGaN, but only a few percent of magnesium dopant atoms are electrically activated and the un-activated dopant atoms do not give rise to free charge carriers.
The epitaxial growth of Group III nitride semiconductor materials on a substrate can be effected by molecular beam epitaxy (MBE) or by chemical vapour deposition (CVD) which is sometimes known as Vapour Phase Epitaxy (VPE).
CVD (or VPE) takes place in an apparatus which is commonly at atmospheric pressure but sometimes at a slightly reduced pressure of typically about 10 kPa. Ammonia and the species providing one or more Group III elements to be used in epitaxial growth are supplied, using a carrier gas, substantially parallel to the surface of a substrate upon which epitaxial growth is to take place, thus forming a boundary layer adjacent to and flowing across the substrate surface. It is in this gaseous boundary layer that decomposition to form nitrogen and the other elements to be epitaxially deposited takes place so that the epitaxial growth is driven by gas phase equilibria.
In contrast to CVD, MBE is carried out in a high vacuum environment. In the case of MBE as applied to the GaN system, an ultra-high vacuum (UHV) environment, typically around 1×10−3 Pa, is used Ammonia or another nitrogen precursor is supplied to the MBE chamber by means of a supply conduit and a species providing gallium and, possibly, indium and/or aluminium and/or a dopant species are supplied from appropriate sources within heated effusion cells fitted with controllable shutters to control the amounts of the species supplied into the MBE chamber during the epitaxial growth period. The shutter-control outlets from the effusion cells and the nitrogen supply conduit face the surface of the substrate upon which epitaxial growth is to take place. The ammonia and the species supplied from the effusion cells travel across the MBE chamber and reach the substrate where epitaxial growth takes place in a manner which is driven by the deposition kinetics.
At present, the majority of growth of high quality GaN layers is carried out using the metal-organic chemical vapour deposition (MOCVD) process. The MOCVD process allows good control of the growth of the nucleation layer and of the annealing of the nucleation layer. Furthermore, the MOCVD process allows growth to occur at a v/III ratio well in excess of 1000:1. The V/III ratio is the molar ratio of the group V element to the Group III element during the growth process. A high V/III ratio is preferable, since this allows a higher substrate temperature to be used which in turn leads to a higher quality GaN layer.
In the growth of p-type GaN or AlGaN by MOCVD the p-type dopant source is typically bis(cyclopentadienyl)magnesium or bis(methylcyclopentadienyl)magnesium; these are also known as Cp2Mg and MCp2Mg respectively.
There have been several reports of the growth of p-type doped GaN by MOCVD such as, for example, U.S. Pat. No. 5,306,662. It has generally been found that the magnesium dopant atoms in magnesium-doped GaN grown by MOCVD are inactive, so that post-growth processing is required to activate the magnesium atoms in order to generate free charge carriers. This is because magnesium atoms are passivated if the growth process is carried out in the presence of hydrogen. Large quantities of hydrogen are present in the growth of GaN by MOCVD (arising from the hydrogen carrier gas, and from the decomposition of ammonia gas if this is used as the nitrogen source), and these tend to passivate magnesium-doped GaN. It has generally been found necessary to activate, the magnesium-doped GaN grown by MOCVD to obtain a reasonable density of free charge carriers, for example by annealing the material or by irradiating the material with a low energy electron beam.
Further disclosures of use of Cp2MG as a p-type dopant in MOCVD growth of GaN or AlGaN are given in U.S. Pat. Nos. 5,831,277 and 5,930,656, and a disclosure of use of Cp2MG as a p-type dopant in plasma-assisted chemical beam epitaxial growth is given in U.S. Pat. No. 5,637,146. EP-A-0 307 995 discloses use of Cp2MG as a p-type dopant in MOCVD growth of GaAs.
U.S. Pat. No. 6,043,140 reports a method of an MOCVD growth process that obtains p-type conductivity in GaN without the need for an annealing step, but this method requires very specific amine gases for the nitrogen source.
At present, growing high quality GaN layers by MBE is more difficult than growing such layers by MOCVD. The principal difficulty is in supplying sufficient nitrogen during the growth process. The two commonly used sources of nitrogen in the MBE growth of nitride layers are plasma excited molecular nitrogen or ammonia.
There have been a number of reports of MBE growth of magnesium-doped GaN that do not require a post-growth annealing or irradiation step and these include U.S. Pat. Nos. 5,657,335 and 6,123,768. The MBE process does not use hydrogen carrier gas, so that the hydrogen level in a MBE growth system is generally lower than the hydrogen level in a MOCVD growth system; as a result passivation of the obtained magnesium-doped GaN is less of a problem in MBE growth than in MOCVD growth. In particular, many reports of MBE growth of GaN use an activated nitrogen plasma source as the nitrogen precursor rather than ammonia, and this eliminates the presence of hydrogen arising from the decomposition of ammonia
M. Mayer et al. have reported, in “J. Cryst. Growth”, Vol 201/202 p318-322 (1999) and in “Proc. 2nd International Conf. on Nitride Semiconductors” (1997), growth of p-type GaN by MBE. Ammonia is used at the nitrogen precursor, and MCp2Mg is used as the source of magnesium dopant atoms. These reports describe neither the growth conditions used nor the p-doping levels achieved. EP-A-1 164 210 discloses MBE growth of undoped GaN at growth temperatures of 850° C. or above.